Charged particle generator yielding a mono-energetic ion beam



R. E. ELMORE 3,514,666

ENERGETIC ION BEAM May 26, 1970 CHARGED PARTICLE GENERATOR YIELDING AMONO- Filed June 16, 1967 2 Sheets-Sheet l INVENTOR. ROBERT E. ELM OREATTORNEY.

y 6, 1970 R. E. ELMORE 3,514,666

CHARGED PARTICLE GENERATOR YIELDING A MONO-ENERGETIC ION BEAM Filed June16, 1967 2 Sheets-Sheet 2 l l r I 2 I I I 2 1 k. l l I 5 o D 1 t D 0 GASI I S 6w L. J ,9

l E N S l O INVENTOR.

ROBERT E. ELMORE Maw ATTORNEY.

United States Patent US. Cl. 315-111 3 Claims ABSTRACT OF THE DISCLOSUREThis charged particle generator produces a highly concentrated beam ofions. An electron gun, at ion chamber, an ion accelerator, and anelectrostatic lens are cascaded and provided with aligned apertureswhereby this ion beam is produced, focused, and directed in awell-controlled manner.

FIELD OF INVENTION AND BACKGROUND Prior art methods of calibratingquadrupole mass spectrometers utilize the Nier gun, which lacks controlover the ion beam which it generates.

The invention is a novel device for the formation, control and focusingof an ion beam. A primary object of the invention is to provide a devicewhich minimizes energy distribution spread of the ion beam. Such spreadis a cause of error in the calibrating of quadrupole mass spectrometers.The invention is of particular utility for such calibration.

Another object of the invention is to provide a novel device forgenerating a high-intensity low-energy positive ion beam havingmono-energetic properties. The term mono-energetic is employed todesignate a single electron volt population atone energy level.

Another object of the invention is to provide a convenient source for alow-energy plasma beam, plasma being understood to be a mixture of ionsand electrons.

GENERAL DESCRIPTION OF DRAWINGS For a better understanding of theinvention, together with other and further objects, advantages andcapabilities thereof, reference is made to the following detaileddescription of the drawings hereto annexed.

In the drawings, FIG. 1 is a perspective view, partially broken away forpurposes of exposition, showing a preferred embodiment of chargedparticle generator in accordance with the invention; FIG. 2 is a view ofsimilar character with reference to a modified form of the invention;and FIG. 3 is a circuit schematic, showing representative electricalpotentials applied to the FIG. 1 embodiment.

DETAILED DESCRIPTION OF THE INVENTION The charged particle generator ionsource herein shown is unique in that it is formed with coaxial elementsand has a well defined electrical region in the physical area in whichion formation occurs. The ion source of the present invention consistsof an arrangement of cylindrical elements located along a common centralaxis as shown in FIG. 1. Each element contains an aperture for thepassage of and control of emitted particles. The ion beam produced isdiscretely defined by a narrow energy distribution about a selectedenergy. The ion source utilizes electron bombardment to produce ions andan electro- 3,514,666 Patented May 26, 1970 static lens to converge theion beam. The ion beam thus generated can be controlled in energy rangefrom less than from (1) electron volt to more than two thousand (2,0 00)electron volts and has a current capacity of zero to one microampere.The charged particle generator of the type herein described must beoperated in a partial vacuum where relatively long mean free pathsprevail, which implies an operation vacuum of 10- torr or greater. Thelimit of travel of a charged particle through a gaseous medium, prior tocollision, is related to the mean free path of the molecules composingthe medium. Temperature of the surrounding medium is not a criticalfactor for this application.

The charged particle gun of either FIGS. 1, 3 or FIG.

2 should be placed in a vacuum chamber 9 (FIG. 3) and evacuated to apressure of 10- torr or greater. The number of ions formed is a functionof the ion chamber pressure; hence, improved ionization efliciency isobtained by admitting a small amount of gas into the ion chamber. Thisimprovement is the result of an increase in the density of neutralparticles in the ion chamber which increases ionization due to theincreased probability of particle collision. The quantity of gasadmitted should not increase the ion chamber pressure more than a decadeabove the surrounding pressure of the chamber (10- torr or so). Thecorrect quantity of gas may be calculated theoreticaL ly but, inpractice, the quantity is determined empirically. The correct quantityof gas and ion chamber pressure are determined to exist at the pointwhere the greatest ion current is obtained.

The elemental gas feed into the ion chamber will determine the type ofion formed and emitted from the chamber. The types of elemental gasesused in this invention and their applications are argon for surfacecleaning (i.e., electron multiplier applications) and nitrogen forrarefied atmospheric simulations. Atmospheric air has also beenintroduced into the chamber, producing a steady flow of mixed positiveions.

The energy of the emitted ions is largely determined by the electricalpotential applied to the ion chamber; thus, the electrical potentialsapplied to the remaining elements of the invention must be related tothe ion chambers potential. Ions formed by the collision of electronswith neutral particles within the ion chamber will have at the point offormation an energy of approximately 0.1 electron volt. This energy isderived from the thermal energy of the neutrals plus that energyobtained from the collision. In addition, the ions will possess, at thepoint of formation, a potential energy equivalent to the electricalpotential existing at the point of formation. Since all points withinthe ion chamber are at the same potential, the chamber is electricallyclosed and, thus, all ions formed within the ion chamber will have thepotential of the chamber which, when added to the thermal and collisionenergies, will yield a group of ions that form a mono-energetic ionbeam.

The filament 11 (FIG. 1) as here employed is of rhenium. In onesuccessfully operative embodiment of the invention it was operated witha current of 5.5 amperes at 2.5 volts. Either alternating current ordirect current may be employed. A tungsten filament is acceptable. Thefilament is biased negatively with respect to the ion chamber by 30 tovolts, in order to assist in controlling the flow of electrons betweenthe filament and the ion chamber. The electron emission controlelectrode 12 (FIG. 1) is in the nature of a metallic thimble or cylinderhaving one end closed with a central aperture. Its function is tocontrol the rate of flow of electrons from the filament to the ionchamber. Therefore, it operates in a manner analogous to the controlgrid of a vacuum tube or cathode ray tube. The flow of electrons dependson the size of the aperture, the physical spacing between the filamentand the electron accelerator 13 (FIG. 1) and the biasing potentials. Theelectron control grid is provided with a variable bias arrangement sothat its voltage may be varied between zero and such negative voltage ascuts off electron flow.

The electron accelerator element 13 is in the form of an invertedthimble or cylinder having one end closed, coaxial with and somewhatlarger in diameter than the element 12 (FIG. 1). It is also providedwith a central aperture. The electron accelerator element 13 has twofunctions. First, it accelerates the flow of electrons into the ionchamber 14 and second, it prevents the escape of ions into thesurrounding vacuum. That is to say, elec trons emitted by the filamentare caused by the electron accelerator to be accelerated away from thefilament and through the apertures of the emission control grid and theelectron accelerator into the ion chamber. The potential of theaccelerator should be from 1 to 5 volts positive with respect to the ionchamber. Increased currents can be obtained by providing two electronaccelerators, as shown in FIG. 2 and later described herein.

Next adjacent the electron accelerator is the ion chamber element whichis cylindrical in form and closed at both ends. Except for centralapertures the ion chamber is provided with a removable gas inlet cap 16(FIG. 1). In the above-mentioned embodiment of the invention the lengthof the ion chamber was approximately fifteen times the distance from thefilament to the near end of the ion chamber.

In operation, gas is introduced into the ion chamber through the gasinlet and the internal pressure of the chamber is increasedapproximately a decade or more above the surrounding vacuum pressure.Parenthetically, it is reiterated that the entire structure of FIG. 1 ispositioned within a vacuum chamber (not shown). The internal pressure ofthe ion chamber 14 (FIG. 1) during operation is approximately 10- torr.Ionization occurs in the chamber due to the collisions between electronsemitted by the filament and the gas molecules. The probability ofcollision between an electron and a neutral gas molecule is related tothe mean free path travelled by the electron and the amount of gaspresent. The high probability of ionization within the chamber is due tothe density of the gas contained within the chamber, the relativelyshort mean free path of the gas molecules and the length of the chamber.Disposed in front of the ion chamber 14 is an ion accelerating element17 (FIG. 1). The remaining elements illustrated in FIG. 1 areelectrostatic focusing elements comprising a focusing electrode 18(FIG. 1) and a ground plane focusing arrangement shown as elements 19,20 (FIG. 1).

The operating potentials indicated in FIG. 3 are furnished by way ofillustration and not of limitation and they were employed with onesuccessful embodiment of the invention.

Element: Voltage 17 170 11 125 20 Ground The wall of 14 8 13 10 18 12 19Ground 12 to 40 In this embodiment the filament 11 was operated atamperes from a 2 volt supply.

The FIG. 2 embodiment differs from that of FIG. 1

in that it has a second electronic accelerating element 15 (FIG. 2).

It should be noted that the invention comprises, essentially, anelectron gun, an electrostatic focusing system, and an ion chamber, allhaving registering apertures which define a straight path for thegeneration, focusing, and direction of an ion beam.

While there has been shown and described what is at present consideredto be the preferred embodiment of the invention, it will be understoodby those skilled in the art that various changes and modifications maybe made therein without departing from the scope of the invention asdefined by the appended claims.

I claim:

1. A charged particle generator yielding a monoenergetic ion beamcomprising in combination:

a substantially closed cylindrical conductive ion chamber having axiallyaligned apertures in opposing ends, said chamber being maintained at afirst electrical potential;

an electron source filament axially aligned with said ion chamber forgenerating free electrons for injection through one of said aperturesand into said ion chamber, said filament being biased negatively withrespect to said ion chamber;

a cylindrical electron control electrode mounted coaxially about saidfilament, said electrode having one end closed with a central aperturetherein and being axially aligned with said ion chamber to control theflow of electrons from said filament to said ion chamber;

an electron accelerator having a central aperture axially aligned withsaid ion chamber and mounted between said ion chamber and said controlelectrode, said electron accelerator being biased positively withrespect to said ion chamber;

means for introducing a gas into said ion chamber wherein ions having auniform energy are formed within said chamber;

electrostatic lens means for focusing ions exhausted out of said ionchamber and into said electrostatic lens means;

an ion accelerator having a central aperture therein mounted betweensaid ion chamber and said electrostatic lens means in axial alignmenttherewith, said ion accelerator being biased negative with respect tosaid ion chamber thereby exhausting ions formed in said ion chamberthrough the other of said ion chamber apertures and into saidelectrostatic lens means; and

said ion chamber, electron source filament, control electrode, electronaccelerator, ion accelerator, and electrostatic lens means mounted alonga common longitudinal axis wherein the ions formed in said ion chamberflow in a direct path to said electrostatic lens whereby the ions arefocused and directed therefrom in a beam with minimum spread.

2. The combination as claimed in claim 1 in which said electronaccelerator is a cylinder having one end closed with a central aperturetherethrough, said cylinder being larger in diameter than said electroncontrol electrode and mounted coaxially therewith wherein said electronaccelerator accelerates the flow of electrons into said ion chamber andfurther prevents the escape of ions from the aperture of said ionchamber;

variable bias means interconnected with said electron control electrodewherein said control electrode may be varied from zero to such negativevoltage as to prevent electron flow to said ion chamber; and

said electrostatic lens further having a focusing electrode biased at ahigher potential than said ion chamber.

3. The combination as claimed in claim 1 in which said electronaccelerator further comprises a split anode.

(References on following page) 5 References Cited UNITED 3,320,457 5/1967 Burdick et a1. 313-82 STATES PATENTS 3,408,519 10/1968 Etlevant etal. 313-231 X Kanmann 313*82 X JAMES W. LAWRENCE, Primary ExaminerBorries et a1 31382 X 5 P. C. DEMEO, Assistant Examiner Smith 313-63 XKing et a1. 313-63 x Eklund 31363 X 313--63, 230; 250 41.9

